1. Field of the Invention
The invention relates to platens, and more particularly to pivoting platens for use in thermal or impact printing devices such as printers, copiers, typewriters and facsimile machines, and more particularly in portable printing devices having a printing device mounted on a carriage, for printing in bands as the carriage is scanned across a copy sheet.
2. Description of Related Art
Ink transfer technology in plain paper printing devices, such as, for example, printers, copiers, typewriters and facsimile machines, is well known. FIG. 1 illustrates a known application, in which fixed-position thermal printhead 30 extends across the full width of sheet 20, which extends into and out of the plane of the Figure. Printhead 30 includes a line of resistors 35, located remote from the edge of the thermal printhead 30, which selectively melt ink on ribbon 25 to form images on sheet 20. Printhead 30 is referred to as a full-width printhead because it extends across the full width of a sheet. Full-width printheads print on a sheet without being moved back and forth across the sheet width.
Low durometer elastomer roll 10 presses ribbon 25 into close contact with resistors 35 and presses sheet 20 into close contact with ribbon 25, ensuring that sheet 20 and ribbon 25 do not slip relative to each other. Roll 10 is driven to rotate in the direction of arrow 15 to advance sheet 20 and ribbon 25 lengthwise past printhead in the direction of arrows 23 and 28, respectively so that printing can occur on the entire length of the sheet.
To achieve uniform image density along the entire width of sheet 20, the pressing forces that compress paper 20 and ribbon 25 between roll 10 and printhead 30 are uniform along the entire width of sheet 20. This is accomplished easily because the surface of printhead 30 that contacts the ribbon is flat and stationary and the pressing roller 10 contains a compressible elastomeric surface.
FIGS. 2-3 illustrate another known application, a variant of which is used in the Citizen PN48 Notebook Printer. This application uses a carriage-type print system in which a printhead containing a line of resistors reciprocates across a sheet to print images in bands on the sheet. The line of resistors are arranged approximately perpendicular to the direction in which the carriage reciprocates. Due to the scanning movement of the carriage, it can be more difficult to maintain uniform pressing forces between the printhead, ribbon and sheet, and, consequently, more difficult to maintain uniform image density across the line of resistors. In the FIG. 2-3 arrangement, thermal printhead 60 scans in scan direction 55 across stationary sheet 80 to produce successive image bands on sheet 80. A line of resistors 70, disposed near the edge of thermal printhead 60, melts ink on ribbon 75 to produce each image band. Ribbon 75 is fed between ribbon supply and take-up rolls 45, which move in direction 55 with printhead 60. After receiving each image band, sheet 80 advances into a proper position for receiving a successive image band.
Spring 65 biases printhead 60 about pivot 63 to press ribbon 75 and sheet 80 against platen 93. Platen 93 includes flat, flexible spring support 90, which supports elastomer pad 85. Spring support 90 is non-rigid, that is, it is designed to be bent or twisted without exceeding the yield point of the material from which it is formed, usually spring steel. Raised bead 98 extends from frame 95 and supports platen 93 for twisting, pivotal movement as printhead 60 scans across sheet 80. The twisting and pivoting of platen 93 on bead 98 accommodates misalignments between printhead 60 and platen 93. Accommodating such misalignments equalizes the pressing forces between printhead 60 and platen 93 across the entire width of printhead 60, resulting in image bands of uniform image density.
The pivoting platen arrangement of FIGS. 2-3 may be suitable when the width of the printhead (which corresponds to the length of the line of resistors) is small, for example, 4 mm in the Citizen PN48 Notebook Printer mentioned earlier. Such a short length, however, requires a large number of carriage reciprocations to print a single sheet of size 81/2 by 11", for example. It is desirable to use wider carriage-type printheads that print wider image bands, and therefore do not require a large number of reciprocations to print a single sheet. This increases print speed, reduces carriage wear (because fewer reciprocations are necessary), and increases the number of pages that can be printed using a roll of ribbon, without increasing the diameter of the roll. For example, the portable copier described in the related U.S. patent applications incorporated by reference above, uses a printhead that prints image bands 24.38 mm wide.
The printing platen arrangement of FIGS. 2-3 has a number of disadvantages when used with wider printheads. In particular, it becomes difficult to insure uniform pressing forces at ends of the printhead. Platen 93 would have to be substantially modified to function properly, i.e., to create uniform pressing forces and thereby ensure uniform image density, in a wide-printhead printing device. Platen 93 could be modified in at least two alternative ways:
1) Vary the thickness across spring support 90 and/or elastomer pad 85, so that the thickness of each is non-uniform. Varying the cross-sectional thickness of support 90 and/or pad 85 would be designed to cause pressing forces at extreme outboard contact points between the printhead and the platen to match pressing forces at the center of the printhead. Such a design, however, would be expensive and difficult to implement due to high manufacturing tolerances that would be required.
2) Design spring support 90 and/or elastomer pad 85 to curl in their relaxed states, so that outboard portions of the printhead would deflect spring support 90 farther at outboard contact points than at central contact points. Such curvature, however, would create at least two problems. First, a curved design would greatly increase manufacturing costs. Second, a curved design would complicate loading a sheet into proper position between the ribbon and the curved platen. Inserting sheets through a space defined by a curved platen on one side and a flat printhead covered by ribbon on the other side would be quite difficult and present an unacceptably high possibility of misfeed.
Because the platen of FIGS. 2-3 and its modifications are unacceptable for wide-band printing devices, as described above, a need has arisen to develop a platen that more efficiently and effectively promotes uniform density of images printed by such printing devices.